With the improvement of the degree of integration of semiconductor integrated circuits, technologies for realizing highly functional, compact information processing devices have been established, and presently, portable information processing devices have become popular in various fields. The use of portable information processing devices, such as IC (Integrated Circuit) cards, cellular phones, PDA's (Personal Digital Assistants) is now so common that each individual user owns such a unit. Since a portable information processing device is thus a device for processing information of a personal level of each user, various kinds of personal information related to privacy are stored in the interior of such devices. Such devices therefore require adequate security measures. Thus in general, a portable information processing device is equipped with a password code checking function and is provided with a security measure that enables use only in the case where, upon checking of a password code that has been registered in the interior in advance with a password code that has been input from the exterior at the time of use, the password codes match each other.
Though a general portable information processing device is equipped with a password code checking function as has been mentioned above, the higher the frequency of use of the device, the more troublesome it becomes to perform the operation of inputting the password code for use each time. Portable information processing devices, which enable switching between a mode that requires checking of a password code for use and a mode that does not require such checking, have thus been proposed. However, conventionally proposed portable information processing devices with a switching function could not ensure adequate security. That is, in order to provide such a switching function, information that indicates whether or not checking is necessary must be stored inside the device. However, if this information becomes rewritten through illegitimate access or becomes rewritten due to some effect even during legitimate use, access may be enabled without checking of the password code even when a user had intended to have set the mode that requires the checking of the password code.